For customers that purchased a new Mac computer since June 11, the upgrade to Mountain Lion is free from Apple.

Apple brags that Mountain Lion includes over 200 new features -- which are obviously too numerous to list here -- but we'll include just a couple of the highlights:

Full iCloud integration

Game Center, Reminders, and Notes -- all three are staples of iOS, and all three are iCloud-enabled

Messages -- another iOS transplant

Notification Center -- most Mac users got their notifications via Growl, but now Apple is getting into the game with the -- once again -- iOS-esque Notification Center

Dictation

Power Nap -- this feature allows a Mac to still periodically check email and sync iTunes content while in a deep state of sleep. If connected to a power source, the machine will also perform Time Machine backups and download software updates.

Facebook and Twitter -- these two social networks are also baked into the upcoming iOS 6 release

A few sites have posted reviews of Mountain Lion and the consensus is that it's a worthy upgrade to Lion. The Verge's Nilay Patel is overall pleased with the upgrade:

Ultimately, this is pretty easy: you should spend the $20 and upgrade to Mountain Lion, especially if you have a newer Mac. You’ll gain a handful of must-have features, and everything will get faster and smoother. I haven’t really missed Snow Leopard at all since upgrading, which is remarkable considering how much I disliked Lion...

Mountain Lion is the first version of OS X to deeply integrate network services at every level, from storing documents to sharing photos to connecting external displays, and it seems that much lighter for it — as though Apple’s relentless charge into its post-PC era has allowed the OS X team to rethink exactly what a PC is and should be. Mountain Lion isn’t perfect, but it’s a confident, thoughtful step towards the future of desktop computing.

Engadget's Brian Heater acknowledges that Mountain Lion is a bargain upgrade, but also notes that it seems as though OS X is mainly taking a back seat to iOS when it comes to attention/features, and that the operating system is in need of a complete "rethink":

That said, it seems time for Apple to make a bold new pronouncement on the desktop front. The company appears to have most of its resources invested in the mobile side -- and there's no question as to why: the iPhone and iPad have reinvigorated the company, making it a computing player on a scale that no one (save, perhaps, for Jobs himself) could have predicted a decade ago. Still, it might be hard for OS X users not to feel neglected -- many of the latest new features feel a bit like iOS hand-me-downs. When and if Apple rolls out a new operating system this time next year, hopefully we'll be seeing a very different side of Mac OS.

But with a price tag of just $20, Mountain Lion is a no-brainer upgrade for members of the Mac community.

Oh no, because when Apple copies, they are allowed to call it inventing remember? As a matter of fact, they just "invented" the 4 inch screen and 4G in a phone with the upcoming iPhone 5. They also "invented" the notification scheme right out from under Android where they copied it from.